Great circle distance on a sphere

This is an online (PHP-)version of a little
TCL/Tk script I slapped together during a stay in Antarctica to
keep track of the line-of-sight
distance to a balloon payload. For some reason I couldn't easily or
quickly find something like this on the web.

So I wrote my own.

This program is about as simple as they come: you enter the
longitude and latitude of two points on the planet (in degrees) and it
tells you the shortest distance between them (the great circle
distance) in kilometer.